Quadrupole Mass Spectrometers

Principal of Operation

A quadrupole field is formed by four electrically conducting, parallel rods(shown in below figure). Opposite pairs of electrodes are electrically connected. One diagonally opposite pair of rods is held at +Udc volts and the other pair at -Udc volts.

                                            

An rf voltage supplies a signal(+V Cos wt) to first pair of rods and (-V Cos wt) to second pair of rods, which appear as oscillating hyperbolic potentials between the four rods. Ions from the ion source are injected into the quadrupole through a circular aperture. As the ions moves down the quadrupole, they undergo transverse motion in x- and y- planes.

The dc electric fields tend to focus positive ions in the positive plane and defocus them in the negative plane. The ions exhibit oscillations with increasing amplitudes until they finally collide with the electodes and become neutral particles. The lighter the ion in mass, the smaller the number of cycles before it is collected by the electrode.

By controlling the ratio Vdc/Vrf, the field can be controlled which allows to pass ions of only one m/z ratio down the entire length of the quadrupole. By simultaneous ramping the dc and rf fields, ions of various m/z ratios are allowed to pass through the mass filter to the detector and an entire spectrum can be produced.

 

The operation of a quadrupole mass analyzer is usually treated in terms of a stability diagram that relates the applied DC potential (U) and the applied RF potential (V(t)) and the RF frequency (omega) to a stable vs. unstable ion trajectory through the quadrupole rods. :

                                  

The ions are passed within a 60^ocone around the axis, therefore does not require focusing slits. Increasing the rod length (5-20 cm) increases the resolution. If the rf frequency is increased, the lenght of the analyzer can be reduced. Rod diameters are also a factor: increasing the rod diameter increases the sensitivity by a large factor, whereas decreasing the diameter increases the mass range.

Time-of-Flight Mass Analyzers

Principal of Operation

A linear-field reflectron allows ions with greater kinetic energies to penetrate deeper into the reflectron than ions with smaller kinetic energies. The ions that penetrate deeper will take longer to return to the detector. If a packet of ions of a given mass-to-charge ratio contains ions with varying kinetic energies, then the reflectron will decrease the spread in the ion flight times, and therefore improve the resolution of the time-of-flight mass spectrometer.



Linear

A time-of-flight mass spectrometer uses the differences in transit time through a drift region to separate ions of different masses. It operates in a pulsed mode so ions must be produced or extracted in pulses. An electric field accelerates all ions into a field-free drift region with a kinetic energy of qV, where q is the ion charge and V is the applied voltage. Since the ion kinetic energy is 0.5mv², lighter ions have a higher velocity than heavier ions and reach the detector at the end of the drift region sooner.

Recall that the kinetic energy of an ion leaving the ion source is:

K.E. = eV

¹/₂ mv² = eV

The ion velocity, v, is the length of the flight path, L , divided by the flight time,t:

n = L / t

Substituting this expression for v into the kinetic energy relation, we can derive the working equation for the time – of –flight mass spectrometer

m/e = 2Vt² / L²

or, rearranging the equation to solve for the time-of-flight: 

Reflectron

The ions leaving the ion source of a time-of-flight mass spectrometer have neither exactly the same starting times nor exactly the same kinetic energies. Various time-of-flight mass spectrometer designs have been developed to compensate for these differences. A reflectron is an ion optic device in which ions in a time-of-flight mass spectrometer pass through a "mirror" or "reflectron" and their flight is reversed.

A linear-field reflectron allows ions with greater kinetic energies to penetrate deeper into the reflectron than ions with smaller kinetic energies. The ions that penetrate deeper will take longer to return to the detector. If a packet of ions of a given mass-to-charge ratio contains ions with varying kinetic energies, then the reflectron will decrease the spread in the ion flight times, and therefore improve the resolution of the time-of-flight mass spectrometer.

Double- Focusing Sector Spectrometers:

Magnetic /electrostatic sector instruments use magnetic and electric fields to disperse ions according to their momentum and translational energy. Like the magnetic sector, the electric sector applies a force perpendicular to the direction of ion motion, and therefore has the form of an arc.  

     

The dependence of mass-to-charge ratio on the electric and magnetic fields is easily derived as below. All ion formed in the ion source are accelerated to a kinetic energy, T of

T = eV = mn² / 2

Solving for the velocity v we get: 

From the Lorentz force law, the magnetic field applies a force evB that must be equal to the centripetal force mv**2/r as the ions move in an arc through the magnetic sector: 

e vB =  m υ ²  / r

Substituting for v, we arrive at the working equation for a magnetic sector mass spectrometer: 

m/e = B²r² / 2V

Therefore ions are accelerated out of the source and are collimated into a narrow beam by a set of slits. As the ions pass through the electrostatic sector, they are dispersed according to their translational energy. Only those ions that have the correct translation energy pass through the slits at the end of the electrostatic sector. Finally the magnetic sector disperses the ions according to their mass to charge ration.

A mass spectrum is obtained by scanning the accelerating voltage (electric field) while hold B constant and scan V  to bring ions with different m/z ratios sequentially to focus at the detector.

Magnetic -Deflection or Sector Mass Analyzer:

In a magnetic deflection mass spectrometer, ions leaving the ion source are accelerated to a high velocity. The ions then pass through a magnetic sector in which the magnetic field is applied in a direction perpendicular to the direction of ion motion. When acceleration is applied perpendicular to the direction of motion of an object, the object's velocity remains constant, but the object travels in a circular path. Therefore, the magnetic sector follows an arc; the radius and angle of the arc vary with different ion optical designs.

 

A magnetic sector alone will separate ions according to their mass-to-charge ratio. Only ions of a single m/z value will have the proper trajectory leading to the exit slit ahead of the detector. By changing the magnetic field strength, ions with differing m/z values are brought to focus at the detector slit.

The ions velocity ‘υ’, in the magnetic field is given by

Limitation:

In a single-focusing magnetic sector instrument there is a lack of uniformity of ion energies, since the accelerating potential experienced by an ion depends on the region at which it is formed in the ionization chamber. The resulting spread in ionic energies produces a spread in their radii of curvature in the magnetic field. The result is peak broadening and low to moderate resolution.

½ m υ ²  =zV

However, the resolution will be limited by the fact that ions leaving the ion source do not all have exactly the same energy and therefore do not have exactly the same velocity.

MALDI (MATRIX-ASSISTED LASER DESORPTION IONIZATION)

Matrix-assisted laser desorption ionization a laser beam is fired at the crystals in the MALDI spot. The matrix absorbs the laser energy and it is thought that primarily the matrix is ionized by this event. The matrix is then thought to transfer part of its charge to the sample molecules , thus ionizing them while still protecting them from the disruptive energy of the laser. Ions observed after this process consist of a neutral molecule [M] and an added or removed ion. Together, they form a quasimolecular ion, for example [M+H]⁺ in the case of an added proton, [M+Na]⁺ in the case of an added sodium ion, or [M-H]^- in the case of a removed proton.

The matrix consists of crystallized molecules, of which the two most commonly used are 3,5-dimethoxy-4-hydroxycinnamic acid, α-cyano-4-hydroxy Cinnamic acid  A solution of one of these molecules is made, often in a mixture of highly purified water and an organic solvent such as acetonitrile or ethanol.

ELECTROSPRAY IONIZATION

In electrospray ionization, a liquid is pushed through a very small, charged and usually metal, capilary. This liquid contains the substance to be studied, the sample, dissolved in a large amount of solvent, which is usually much more volatile than the sample. Volatile acids, bases or buffers are often added to this solution too. The sample exists as an ion in solution either in its anion or cation form. Because like charges repel, the liquid pushes itself out of the capillary and forms an aerosol, a mist of small droplets about 10 μm across.

The aerosol is at least partially produced by a process involving the formation of a Taylor cone and a jet from the tip of this cone. An uncharged carrier gas such as nitrogen to evaporate the neutral solvent in the droplets. As the solvent evaporates, the analyte molecules are forced closer together, repel each other and break up the droplets. This process is called Coulombic fission because it is driven by repulsive coulombic forces between charged molecules. The process repeats until the analyte is free of solvent and is a lone ion. In electrospray processes, the ions observed may be quasimolecular ions created by the addition of a proton or cation(Na⁺).

FAST-ATOM BOMBARDMENT (FAB)

In FAB a high-energy beam of netural atoms, typically Xe or Ar, strikes a solid sample causing desorption and ionization. It is used for large biological molecules that are difficult to get into the gas phase. FAB causes little fragmentation and usually gives a large molecular ion

peak, making it useful for molecular weight determination.

The atomic beam is produced by accelerating ions from an ion source though a charge-exchange cell. The ions pick up an electron in collisions with netural atoms to form a beam of high energy atoms.

CHEMICAL IONIZATION

Chemical Ionization (CI) is especially useful technique when molecular ion is not observed in EI mass spectrum, and also in the case of confirming the mass to charge ratio of the molecular ion.

Chemical ionization involves ion-molecular chemical interactions between the sample molecules and a reagent gas like CH₄ , NH₃ , i-C₄H₁₀.  The pressure of the chamber is maintained at 0.1 – 1 torr.

Reagent gas molecules are present in the ratio of about 100:1 with respect to sample molecules. Positive ions and negative ions are formed in the CI process. Depending on the setup of the instrument (source voltages, detector, etc...) only positive ions or only negative ions are recorded.

In CI, A reagent gas is ionized by electron impact ionization in the source with energy up to 200-500 eV to give ionized reagent gas molecules.

CH₄ + e^- -----> CH₄^+. + 2e^- ------> CH₃⁺ + H^.   (electron impact)

i-C₄H₁₀ + e^- -----> i-C₄H₁₀^+. + 2e^-            (electron impact)

NH₃ + e^- -----> NH₃^+. + 2e^-               (electron impact)

Primary reagent ionization is followed by second order process in which the primary ion reacts with additional reagent gas molecules to produce a stabilized reagent ion.

Methane:

CH₄^+. + CH₄ -----> CH₅⁺ +CH₃^.       (secondary ions)

CH₄^+. + CH₄ -----> C₂H₅⁺ + H₂ + H^.     (secondary ions)

Isobutane:

i-C₄H₁₀^+. + i-C₄H₁₀ ------> i-C₄H₉⁺ + C₄H₉ +H₂   (secondary ions)

Ammonia:

NH₃^+. + NH₃ ------> NH₄⁺ + NH₂^.    (secondary ions)

NH₄⁺ + NH₃ ---------> N₂H₇⁺     (secondary ions)

 

Ion molecule reactions occur between ionized reagent gas molecules (G) and volatile analyte neutral molecules (M) to produce analyte ions. Pseudo-molecular ion MH⁺ (positive ion mode) or [M-H]^- (negative ion mode) are often observed. Unlike molecular ions obtained in EI method, MH⁺ and [M-H]^- detection occurs in high yield and less fragment ions are observed.

Positive ion mode:

GH⁺ + M ------> MH⁺ + G

Negative ion mode:

[G-H]^- + M ------> [M-H]^- + G

i.e.,

Methane:

CH₅⁺  + M    ------à  [M+H]⁺ +CH₄

CH₅⁺  + M    ------à  [M+ CH₅]⁺

C₂H₅⁺ + M    ------à  [M+ C₂H₅]⁺

Isobutane:

i-C₄H₉⁺ + M ----à [M+H]⁺ + C₄H₈

Ammonia:

NH₄⁺ + M ---à [M+H]⁺ +NH₃

NH₄⁺ + M ---à [M+NH₄]⁺

These simple proton transfer reactions are true gas-phase Acid-Base processes in the Bronsted-lowrey sense.

A"tight" ion source (pressure=0.1-2 torr) is used to maximize collisions which results in increasing sensitivity. To take place, these ion -molecule reactions must be exothermic.

Proton transfer is one of the simple processes observed in positive CI:

RH⁺ + M -----> MH⁺ + R

One of the decisive parameter in this reaction is the proton affinity. For the reaction to occur, the proton affinity of the molecule M must be higher that the one of the gas molecule.

Choice of reagent gas:

Two factors determine the choice of the gas to be used:

  Proton affinity PA

  Energy transfer
 

 Choice of reagent gas affect the extend of fragmentation of the

quasi-molecular ion.

In methane positive ion mode CI the relevant peak observed are MH+, [M+CH₅]⁺, and [M+C₂H₅]⁺; but mainly MH⁺

In isobutane positive ion mode CI the main peak observed is MH⁺.

In ammonia positive ion mode CI the main peaks observed are MH⁺ and [M+NH₄]⁺.

  Thus molecular information is obtained from protonation of sample molecules and the observed m/z values is one unit greater that that of the molecular ion is known as quasi molecular ion.

The internal energy of MH⁺ produced from CH₅⁺ , i-C₄H₉⁺ , NH₄⁺ is in the order

CH₅⁺ > i-C₄H₉⁺ > NH₄⁺

Therefore NH₃ (ammonia) is the most used reagent gas in CI because of the low energy transfer of NH₄⁺ compare to CH₅⁺ for example. With NH₃ as reagent gas, usually MH⁺ and MNH₄⁺ (17 mass units difference) are observed.

 

Electron Impact

The electron impact ionization is highly developed and most commonly used ionization method in mass spectrometer. In Electron Impact ionization the chamber is maintained at a pressure of 0.005 torr.

 The gaseous neutral sample molecules enter into the ion chamber through the molecular leak. A tight helical beam of highly energetic accelerated electron beam (70ev) from a glowing tungsten or Rhenium filament pass perpendicular to the incoming gas molecule.
These electrons are drawn off by a positive charged slit called as electron trap, which is on the opposite side of the filament. Thus electrons travel across the ion chamber. Ions are generated by the exchange of energy during the collision of the electron beam and sample molecules. As the electron beam is highly energetic (70 ev), it provides sufficient energy to gas molecules to have ionization and to cause the characteristic fragmentations of sample molecules either by loss of radicals or by loss of neutral molecules.

                                                                    e^- (70eV)

M ------------------> M^+. + 2е^-

M ^{+ .}------------> A⁺   + B ^.

M ^{+ .}  ---------------> A^{+ .}   + B

The positive ions formed in the ion chamber are drawn out by a small electrostatic field between the larger repeller plate (positively charged) and the first accelerating slit (negatively charged).
A strong electrostatic field between the 1st and 2nd accelerating slits of 400-4000 V, accelerates the ions of various masses to their final velocities. The ions emerge from the final focusing slit
(0 volts) as a collimated ion beam with velocities and Kinetic Energies as

                                  zV = ½ m₁ υ ₁ ² = ½ m₂ υ ₂ ² = ½ m₃ υ ₃ ² = ………….

Where

 z = Charge of the ion

 V = Voltage

 υ = Velocity of various ions

 m = Mass of respective ion

Then the collimated ion beam will enter in to the Ion-Analyser, which sort the ions as per their m/z ratios.